This invention relates, in general, to electronic devices, and more particularly, solid state sensor devices.
Solid state sensor devices including silicon-based pressure sensor devices are well known and are used, for example, in automotive, industrial, and biomedical applications. A significant number of applications are becoming important where pressure sensor devices are exposed to harsh environments. Such environments include solvent mixtures (e.g., fuels), water, salt water, acids, and bases. Users have found that standard pressure sensor devices fail in such environments. For example, solvents can cause swelling and/or dissolution of polymeric packaging materials; salt water, acids or bases can corrode metallic surfaces; and water, acids, and bases can etch silicon surfaces.
Manufacturers have used several approaches in an attempt to achieve media compatibility. In one approach, fluorosilicone gels are used to protect the sensor device, wirebonds, portions of the package, and leads. Fluorosilicone gels have several disadvantages including an incompatibility with fuels (e.g., swelling).
In another approach, manufacturers use organic coatings (e.g., parylene) to protect those surfaces of the sensor exposed to the harsh media. The organic coatings are used either alone or together with fluorosilicone gels. Organic coatings have several disadvantages including poor process throughput (i.e., low units/hour), negative effects on electrical parameters, and delamination, which can lead to corrosion failures. Additionally, complicated adhesion promotion process techniques are required to prevent the organic coatings from delaminating, which adds to process cycle time and cost.
In still another approach, the back side or the side that does not contain any electronic devices is the only side exposed to the harsh media. This approach is suitable for differential pressure sensor devices with harsh media on the back side of the diaphragm only but not for absolute pressure sensor devices unless a topside constraint is used. With the topside constraint approach, a constraint substrate is attached to the sensor device to provide a hermetically sealed chamber with the lower side of the sensor being exposed to the harsh media. One disadvantage of this approach is that once the constraint substrate is attached to the sensor device, laser trimming of resistors present on the sensor device is not practical.
As is readily apparent, media compatible sensor structures and methods are needed that overcome the disadvantages of the prior art. It would be of further advantage to have cost effective structures that utilize proven processing techniques.